Replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism

Abstract

LIMESTONES are an important source of palaeomagnetic data, with magnetite the dominant carrier of the magnetization. Because the magnetite is present in low concentrations, however, it has been directly observed only rarely and in trace amounts in acid-treated separates. Such residues are composed primarily of spheres, the ultimate origin of which has been enigmatic⁷. Yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite, which has been inferred to carry either a chemical or a viscous remanent magnetization. Here we present electron microscope data obtained from a sequence of samples of Onondaga Limestone from an east–west traverse across northern New York, which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite. Remagnetization is thus caused by chemical, rather than viscous, processes. This chemical remanent magnetization is compatible with a fluid-mediated event occurring on a regional scale, induced by tectonic stress of Alleghenian (late Palaeozoic) age.